An AC feeding circuit is suitable for long-distance and large-capacity power supply, and therefore is used for power supply for bullet trains and others. An electric railway is a single-phase load. Therefore, generally, in AC traction substation, a three-phase to two-phase transformer transforms three-phase power received from electric power companies, etc., into one pair of single-phase powers having a phase difference of 90 degrees (e.g., see Non-Patent Literature 1). As the three-phase to two-phase transformer, a Scott-T transformer (receiving voltage: 66 kV˜154 kV) or a modified Woodbridge transformer (receiving voltage: 187 kV˜275 kV) is used to avoid unbalance of three-phase power supply.
In addition, in order to address a voltage fluctuation, the AC traction substation is equipped with a static VAR compensator (SVC). The SVC not only makes compensation for reactive power (VAR compensation) but also regulates active power by using inverters.
FIG. 11A is a connection diagram showing an SVC installed at feeding side in the Scott-T transformer in a conventional AC traction substation. This SVC is typically called railway static power conditioner (RPC). R-phase, S-phase, and T-phase represent inputs at three-phase side in the Scott-T transformer, respectively. Main-phase and Teaser indicate two single-phase powers formed by the Scott-T transformer, respectively. The power conditioner (RPC) 2a includes inverters 6m and 6t connected to feeders 4m and 4t in the Main-phase and the Teaser, respectively, and a DC capacitor 20 provided between and connected to the two inverters 6m and 6t. Electric trains 8m and 8t run by using electric powers supplied to the feeders 4m and 4t in the Main-phase and the Teaser, respectively, as power sources.
In the Scott-T transformer, if a load in the Main-phase and a load in the Teaser are balanced, a load at the three-phase side is also balanced. In addition, a voltage fluctuation is less when reactive power is low. Accordingly, the power conditioner (RPC) 2a is configured in such a manner that the inverters 6m and 6t connected to the Main-phase and Teaser, respectively, make compensation for the reactive power, and these two inverters mutually accommodate ½ of a difference between active powers in the two single-phase powers to equalize the active power in the Main-phase and the active power in the Teaser, thereby balancing the load at the three-phase side.
FIG. 11B is a connection diagram showing an SVC installed at the three-phase side in the Scott-T transformer. FIG. 11C is a connection diagram showing an SVC in which inverters are installed in Main-phase and Teaser in a scalene Scott-T transformer in which Main-phase and Teaser in Scott-T transformer are directly connected to each other and single-phase feeding by Slant-phase is performed, and these two inverters are connected to each other by a DC circuit. The SVC shown in FIG. 11B is called “three-phase SVC.” The SVC shown in FIG. 11C is also called single-phase feeding power conditioner (SFC). A three-phase SVC 202a includes an inverter 6 and a DC capacitor 20 connected to the inverter 6, while an SFC 402a includes inverters 6m and 6t, and a DC capacitor 20 connected to the inverters 6m and 6t. The inverters 6, 6m and 6t make compensation for the reactive power and regulate the active power.
In AC feeding, in a single-phase circuit extending from an AC traction substation to a sectioning post, a phenomenon (problem) occurs. That is, a voltage drop occurs by running of an electric train, due to resistance or reactance of railway impedance in a feeder, and thereby a desired feeder voltage at a terminal end of the feeder is not attained. To obviate the phenomenon (problem), Patent Literature 1 discloses a feeder voltage regulator at a terminal end of a feeder to make compensation for a voltage fluctuation at the terminal end of the feeder by using an interactive converter (inverter) connected via an interactive transformer at the terminal end of the feeder. The feeder voltage regulator is configured in such a manner that a battery for making compensation for a long-term fluctuation in active power and a DC capacitor for making compensation for a short-term fluctuation in the active power are connected in parallel at DC side of the interactive converter (inverter).